A silicon-on-insulator (SOI) substrate is a silicon substrate on which a silicon dioxide thin film, referred to as a buried oxide film, is formed, and in addition, a silicon thin film, referred to as a silicon active layer, is formed thereon. The SOI can be used as a substrate of an optical circuit; by etching the silicon active layer, which is the uppermost layer, into a wire shape, it is possible to form a wire waveguide that has a wire-shaped silicon as a core and has both an underlying buried-oxide film and overlying air as cladding surrounding the core. The over-cladding over the core may additionally be substituted by silicon dioxide. (While light is guide along the waveguide, electromagnetic field of the light is distributed so as to penetrate into the cladding from the core as a center. Therefore, the “waveguide” includes not only the core but also the cladding, and the “wire” refers to only the core of the waveguide. In addition “core width” refers to a distance between lateral sides of the core in a cross-section perpendicular to a direction of wave-guiding, and “core height” refers to a distance between a top and bottom faces of the core in a cross-section perpendicular to the direction of wave-guiding. A similar situation applies in the following description.)
Combining micro elemental optical devices that have various basic functions and integrating them on a SOI substrate can produce optical circuits. Most elemental optical devices for optical circuits are made of waveguides, which makes miniaturization of the devices relatively easy. Basic elemental optical devices are optical waveguides themselves, such as linear waveguide, bent waveguide, branched waveguide. By combining a plurality of these waveguide parts, elemental optical devices such as directional couplers and interferometers can be constructed, and furthermore, by combining wavelength filters with these, elemental optical devices such as a wavelength multiplexers/demultiplexers and optical switches can be constructed.
Principal components of optical circuits are optical waveguides, but it has to be noted that cross-sectional shapes of cores of waveguides that are preferable to various optical devices are not necessarily the same. An important characteristic of optical waveguide used for optical wiring is its optical loss being small. When it is etched into its wire shape, the silicon core suffers side wall roughness, which causes optical scattering loss. Therefore, it is desirable that the height of the core of a linear waveguide is small so that the area of the walls can be small.
However, if the thickness of the core is too thin (the height is too low), mode field becomes large, which will result in increase of propagation loss rather than decrease, of a bent waveguide. Since an optical device such as directional coupler or the like includes many bent waveguides, in the case of optical circuits whose integration has to be high, a thick core is better. The thickness of a silicon active layer of an SOI substrate is uniform. Therefore, in conventional technology, considering the trade-off between reduction of optical loss and improvement of integration level of the whole optical circuit, it is necessary to select the height of the core of the waveguide of the whole optical circuit. Furthermore, the width of the core is selected so that the optical waveguide holds a single mode.
Operational characteristics of elemental optical devices depend also on direction of polarization. An optical waveguide formed on the substrate normally has waveguide modes of a TE mode, which is dominated by electrical fields with a direction parallel to the substrate and a TM mode, which is dominated by electrical fields with a direction perpendicular to the substrate, which is caused by optical symmetry of the waveguide with respect to direction of thickness and direction of width. In general, distribution of the electromagnetic field of guided light in the cross-section perpendicular to the direction of propagation, that is, mode field, differs in size and form between the TE mode and the TM mode. As a result, operational characteristics of each of the elemental optical devices in the TE mode and the TM mode differ; one may exhibit a high performance in the TE mode, and another may exhibit a high performance in the TM mode. However, in the conventional technology, since it is difficult to have direction of polarization rotate at will, there has been a problem that performance deterioration (for example, optical loss) is caused in specific elemental optical devices in an optical circuit.
Patent Document 1 discloses a converter that rotates direction of polarization continuously, and also changes the height and width of the core. However, with regard to the converter disclosed here, since core material in the form of a rectangular solid or a wedge shape is stacked up, there is a problem that it is difficult to form in a layered substrate and to continuously change cross-sectional shape of a waveguide.
[Patent Document 1]
JP Patent Kokai Publication No. JP-P2006-509264A